F/R gear ratio for 1/8 on-road conversion

[simplechamp]

I am currently converting my Jammin buggy to an on-road GT car.

These are the combination of F/R diff gears I currently have to choose from: spiral cut 10/45, standard cut 13/43 and 13/45.

What would you choose, and why? Thanks for any opinions.

[BrianG]

I look at this in two ways:

1) Spiral vs straight cut. Spiral cut gears have more contact area to reduce stress on individual teeth and are quieter, but they do place more thrust load on the bearing (under heavy load, tends to "push" against the inner race of the bearing). Personally, the reduced tooth stress of spiral cut is offset by the fact your bearing may wear out faster, which loosens the mesh. If there was a way to use a thrust bearing to keep the pinion gear from stressing a normal bearing, I would choose spiral cut style.

2) There are a couple considerations for the gear ratio. First is the size of the pinion. Smaller pinions have more wear since they turn more. But since both pinions above are the same size, this doesn't matter. The ring gear with fewer teeth will technically have more wear than the other, but I don't think the difference is worth considering.

The second consideration is how much gearing you have with the spur/pinion and/or transmission (none for buggies). If you find you are limited in spur/pinion gearing choices (maybe due to motor size or spur availability), then you choose the F/R diff that complements those limitations.

Other than that, I would say the choice is arbitrary.

[simplechamp]

I currently have a Slipperential with 46T steel spur, but I think I might try one of the Delrin ones, and if I do there is 46T and 50T.

I guess I'll start out with 43/13 standard cut with 46T center spur and go from there. I was mainly wondering if an on-road GT needed more or less gear reduction than an off-road buggy.

[BrianG]

Well, that depends on the top speed you want to hit. Plug some numbers into the calc and see what you get (parameter solver may help more here).

Just keep in mind any gearing limits (motor size, motor mount adjustment, etc), and allow room for further tweaks (in other words, don't get a ratio that will put you at the upper/lower limit of spur/pinion choices).

[simplechamp]

I'm not looking to go too fast, I usually gear for 40mph, but maybe since this is an on-road car I'll be a bit more tempted for speed runs and go for 45-50mph, but that would be max. I'll still probably start at 40.

I guess I'll just leave the gearing the same as I have it in the buggy now (43/13 F/R, 46/16 C, 4S lipo). I currently have an 1860kv motor, but will be changing to a 2050kv Tekin buggy motor to get closer to 30k RPM and also utilize the sensored operation.

[brian015]

I'm planning to do a similar on-road buggy conversion. Plugging the numbers in the calculator (I had to guess on tire diameter because I don't have any - but assumed they'd be a little smaller than my off-road buggy tires) made me realize that I should pick up a CC 2650kv motor (instead of the 1800kv I use in my off-road buggies). I'm going to run standard 43/13 buggy diffs and 46t spur on 4s. This should give me good gearing options from 40-50mph.

For example:
Differential Ratio: 3.3076923076923075
Transmission Ratio: 1
Other Ratio: 1
Spur Tooth Count: 46
Pinion Tooth Count: 15
Total Voltage: 14.8
Motor KV: 2650
Tire Diameter (inches): 3.7
Tire Ballooning (inches): 0
Motor Current Draw: 0
Motor Coil Resistance: 0.0035
Spur/Pinion Ratio: 3.07 : 1
Total Ratio: 10.14359 : 1
Tire Circumference (inches): 11.62 inches (295.25 mm)
Rollout: 1.15:1
Total Motor Speed: 39220 RPM
Vehicle Speed: 42.56 mph (68.37 km/h)
Effective KV Value: 2650
KT constant: 0.51 oz-in/A
Motor Torque: Amperage not specified...
Final Torque: Amperage not specified...
Final Power: Amperage not specified...

[snellemin]

The ofna GTP has 44/14 gearing in the diffs.

You don't need all that gear reduction for onroad. The 4 pole motors make more then enough torque for onroad.